1. Field of the Invention
The present invention relates to ultrasonic motors comprising a stator and a mobile element and, more particularly, to selecting appropriate materials for the stator and the mobile element.
2. Description of the Related Art
Conventional ultrasonic motors can obtain a high torque during low speeds and are used in a variety of mechanisms. For example, many cameras use conventional ultrasonic motors. Conventional ultrasonic motors consist of a stator in pressure contact with a mobile element. The stator includes an electro-mechanical conversion element and an elastic body. Electrical signals are supplied to the electro-mechanical conversion element and, in response, the electro-mechanical conversion element generates progressive oscillations or vibrations in the elastic body. The mobile element is in pressure contact with the elastic body so that the vibrations in the elastic body act to drive the mobile element via friction between the elastic body and the mobile element. The electro-mechanical conversion element of the stator is typically a piezoelectric material. The performance of conventional ultrasonic motors is largely determined by the part of the stator ("stator contact part") and the part of the mobile element ("mobile element contact part") which are in contact with each other. The stator contact part is typically the elastic body of the stator. Consequently, the selection of the materials for the stator contact part and the mobile element contact part is particularly important. The stator contact part typically consists of metallic materials such as aluminum, steel and stainless steel. The mobile element contact part typically consists of plastic materials.
However, various problems occur in convention ultrasonic motors due to the materials of the stator contact part and the mobile element contact part. For example, the oscillations generated in the elastic body of the stator are not always efficiently converted to a drive force for driving the mobile element, thereby resulting in an increased power consumption of the ultrasonic motor. Moreover, the stator contact part and the mobile element contact part can undergo chemical changes (such as hydrolysis), thereby resulting in the stator and the mobile element becoming firmly affixed to each other. Further, abrasive particles can adhere to the stator contact part and the mobile element contact part, thereby resulting in deteriorating characteristics and reduced performance of the ultrasonic motor.